1. Field of the Invention
The present invention generally relates to cable seal assemblies and methods for sealing electrical, optical, and hybrid cables.
2. Related Art
The problem of sealing to long, thin objects such as cables when an end is not accessible has not been effectively solved. Because of this, long cables, such as installed cables, often cannot be repaired without cutting them.
Shafts, cables and the like which extend through bores, such as cable ends extending into underwater connector halves or into an equipment housing or junction box, are sometimes sealed by means of an O-ring seal seated in an annular groove on the inner surface of the bore through which the cable extends. Such o-ring seals to cables often fail for the following reasons. O-ring seals have a relatively small axial contact area with the opposing surfaces of the shaft and bore. This area may be insufficient to allow the O-ring to bridge over irregularities on the shaft and bore surfaces, and requires high seating pressures to make sure that the seal is effective. Because of the high seating pressures required, O-rings often cannot be used to effectively seal against the outer jackets of most submarine or subsea cables. Generally, cables such as these have outer jackets and/or cores of polyethylene or similar plastic material. When exposed to the seating pressures of O-ring seals over time, the cable deforms and the seals leak. O-ring seals are primarily intended to be used against rigid, smooth surfaces.
When an external pressure Pa is applied to a properly seated and lubricated O-ring seal, the seal slides in its seat or groove until it meets the wall of the groove on the side opposite to the applied pressure. External pressure Pa exerts an unseating pressure tending to separate the seal from the opposing surfaces of the bore and cable. But the pressure available to hold the seal against these surfaces is equal to the applied pressure Pa plus the pressure Pc exerted against the shaft and bore by squeezing the seal between them, i.e. Pa+Pc, and is therefore always greater than the unseating pressure. This means that the seal remains seated regardless of the applied external pressure. At high enough pressures, the energized O-ring deforms into the small crevice between the shaft and bore, and this can eventually cause seal failure.
Unstressed O-ring shaft seals have a round cross-section. Elastic shaft seals of rectangular or square cross-section are also known, such as the so-called Morrison-type seal described in a paper entitled “An Investigation of Cable Seals”, by J. B. Morrison, Applied Physics Laboratory, University of Washington, Report #54-41, Mar. 1, 1954. Use of such a seal in a cable sealing arrangement is described in U.S. Pat. Nos. 5,873,750 and 6,067,395 of Cairns et al. This type of seal has a larger contact area with the shaft and cable than a corresponding round cross-section shaft-type seal, but otherwise employs the same physical operating principles. It was assumed by Morrison and others that this type of seal had to have an inner diameter substantially smaller than that of the shaft over which it is installed, and simultaneously a larger outer diameter than the cavity housing it. This resulted in a seal which was often difficult to install or produced disproportionately high squeeze.